1. Field of the Invention
The present invention relates to a frequency-selective oscillator for outputting two difference frequencies, and more particularly to a compact frequency-selective oscillator capable of obtaining a stable output voltage.
2. Description of the Related Art
Conventional oscillators will be described with reference to FIGS. 10 and 11. FIG. 10 is a schematic diagram showing circuitry and operation timing of a first conventional oscillator, and FIG. 11 is a schematic diagram showing circuitry and operation timing of a second conventional oscillator.
As shown in FIG. 10, the first conventional oscillator is composed of a crystal resonator 1 oscillating a constant frequency, and an oscillation circuit 2 formed as an oscillator IC for tuning and amplifying the frequency from the crystal resonator 1. In this structure, a frequency-switching voltage (SEL) arbitrarily switchable between a high level (High) and a low level (Low) is input to a selector terminal of the oscillation circuit 2, and a supply voltage (Vcc) is input to a switching logic control terminal of the oscillation circuit 2.
In the above-mentioned structure, the first conventional oscillator operates as shown in the operation timing of FIG. 10. In other words, when the frequency-switching voltage (SEL) from the selector terminal is High, an output signal Q of a specific frequency and an inverted signal of Q (inverted Q) are output, while when the frequency-switching voltage is Low, Q and inverted Q are not output.
The second conventional oscillator shown in FIG. 11 is also known as another conventional oscillator.
As shown in FIG. 11, like the above-mentioned first conventional oscillator, the second conventional oscillator is composed of a crystal resonator 3 and an oscillation circuit 4, in which the frequency-switching voltage (SEL) is input to the selector terminal of the oscillation circuit 4, and the switching logic control terminal of the oscillation circuit 4 is connected to ground (GND).
Then, in the above-mentioned structure of the second conventional oscillator, when the frequency-switching voltage (SEL) from the selector terminal is High, Q and inverted Q are not output, while when the frequency-switching voltage is Low, Q and inverted Q are output.
In other words, the first conventional oscillator and the second conventional oscillator are reverse to each other in terms of output timing in response to the same frequency-switching voltage.
As a conventional technique for a crystal oscillator switching and outputting a plurality of frequencies, there is Japanese Patent Application Laid-Open No. 2005-142966 (Applicant: Nippon Dempa Kogyo Co. Ltd.; Inventors: Hiroshi Yoshida and Minoru Fukuda) laid open on Jun. 2, 2005, and entitled “Multi-Frequency Switching Type Crystal Oscillator.”
In this conventional technique, the crystal oscillator includes a plurality of crystal resonators different in oscillation frequency from each other, a plurality of oscillation circuits, and a switching unit for selectively switching the oscillation circuits, in which the crystal resonators consist of a plurality of oscillation areas provided on a single crystal blank, the oscillation circuits and the switching unit are integrated in an IC chip, and the crystal blank and the IC chip are hermetically enclosed in the same container. This structure makes it possible to provide a compact, surface-mounted, multi-frequency switching oscillator.
As other conventional oscillators, there are disclosed in published Japanese translation No. 2001-523908 of a PCT international publication and Japanese Patent Application Laid-Open No. 2004-070962.
However, the first conventional oscillator and the second conventional oscillator can both output only a specific frequency, determined by the crystal resonator and the oscillation circuit, and its inverted frequency. Therefore, in order to output two different frequencies, two conventional oscillators are needed as separate devices, resulting in a problem of hindering the downsizing of the entire device.